Treatment of steam systems



jy gw. gli@ v G. B. HATCH TREATMENT oF STEAM sys'rms Filed Oct. 17, 1945mmmI w w WWA-@ mf ATrRNEY atented '.iiiiy 3i, 95i

2,562,549 TREATMENT or STEAM SYSTEMS George B. Hatch, on Park, Pa.,assigner to Hall Laboratories, Inc., Pittsburgh, lPa., a corporation ofPennsylvania Application October 17, 1945, Serial No. 622,736

(Cl. 2ML-23) 8 Claims.

This invention relates to a method of prevent- I ing or retardingcorrosion of steam systems.

More specically stated, the invention relates to a method of preventing,inhibiting, or retarding corrosion of metallic surfaces coming incontact with steam vapor or condensate in such systems.

The deterioration of metallic surfaces in a steam system is greatlyaccelerated by acidic conditions and/or the presence of oxygen. Acidiccorrosion in the boiler is easily eliminated by alkali treatment of theboiler feed-water, but this does not insure the absence of acidiccorrosion in the condensate system which is caused by dissolved carbondioxide. Carbon dioxide enters the system in several different ways.Since it is a constituent of air, any air entering the system bringswith it a certain amount of carbon dioxide. Carbon dioxide is dissolvedin the make-up Water which is added to the system. It is also adecomposition product of carbonates and bicarbonates present in theboiler water. The water employed in steam systems also often containsdissolved oxygen. When the Water is heated to form steam, the dissolvedoxygen is released from the water and mixes with the steam. Oxygen alsomay enter steam systems by in-leakage when the internal pressure is lessthan that of the atmosphere. When the steam contains a mixture ofdissolved oxygen and carbon dioxide, this combination becomes extremelycorrosive as the steam condenses on metal surfaces.

A Generally stated, an object of this invention is to provide a methodof treatment whereby the steam may be permeated with a volatilized amineto render the steam condensate materially less corrosive than it wouldotherwise be, and under optimum conditions to make such condensatenon-corrosive.

More specifically, it is an object of this invention to provide aprocess whereby acid-reacting water soluble salts of certain amines maybe introduced at some appropriate place into a steam system, preferablynear the terminal of the condensate return line, or between suchterminal and the boiler.

Other objects of the invention and the manner of practicing the samewill be apparent from the following description taken in conjunctionWith the single figure in the drawing, in which a steam system isdiagrammatically illustrated with legends to indicate the various partsthereof.

It is common practice to treat boiler water with alkaline materials suchas caustic soda, and

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consequently there is little likelihood of acidic corrosion in theboiler. However, the alkaline alkali-metal salts and hydroxides do notas a rule carry over with the steam if the boiler is operated eiciently.Therefore, that part of the sys-V tem which is in contact withcondensate from the steam does not receive the benet of thealkalinecompound which has been added to the boiler and may be exposed to acidicconditions. Corrosion, accordingly, may occur in the steam line, thecondenser, and the condensate return lines, unless preventive steps aretaken. Dreyfus in U. S. Patent 2,053,024 discloses the use of certainwater-soluble alkaline volatile amines which have been found effectivein preventing or reducing acidic corrosion by steam condensate in thoseparts of the system which have been mentioned.

In practicing this invention, the amine is dlssolved in the boilerwater, volatilized with the steam and condensed with the condensate sothat an alkaline environment is maintained in. all parts of the system.Upon leaving the boiler,

the volatile amine reacts with carbon dioxide and carbonio acid to formboth amine carbonates and amine bicarbonates. If a slight excess ofamine beyond that amount required to neutralize the acidic compounds iscarried into the condenser, some free amine in an uncombined state willow through the system.

Generally speaking, most of the commercially Navailable volatile amineswhich are used in practicing the Dreyfus invention are relativelydithcult to handle inasmuch as their vapors are extremely pungent andobnoxious, and if any of these particular amines come in Contact withthe human skin considerable irritation is observed. The storage andhandling of the volatile amines are matters which require care since thematerials are inflammable. 'I'he salts of the volatile amines, which Iemploy, are free-flowing solids rather than liquids, consequently theyhave very low vapor pressures and have practically no odor. In addition,they present no iire hazard, are easily handled, and if they should comein contact with the skin, no appreciable injury is done.

In most cases, some part of the steam system between the terminal of thecondensate return line and the boiler is open to the atmosphere. Thecondensate may discharge into an open sump or hot-well, or a de-aeratingor open feedwater heater may be installed for the dual purpose ofremoving air from the system and heating the feed-water before it entersthe boiler. Since both the amine bicarbonate and amine carbonate arefairly unstable, it is not unusual to ilnd that in passing through ahot-Well, or deaerating heater they are decomposed into free amine andfree carbon dioxide. A considerable portion of any uncombined amine maythereby escape from the system through such an opening under theseconditions. Although it is'extremely desirable to remove as much carbondioxide from the system as possible, the loss of volatile amine isundesirable as it should be retained for recombination with additionalcarbon dioxide which subsequently enters the system.

In accordance with the process of my invention, an acid-reacting aminesalt is added to the system at any convenient point between the terminalof the condensate return line and the boiler. Where a de-aerating heateris installed ahead of the boiler, the addition should be made to thewater as it enters the heater or at some convenient point in thecondensate return line preceding the heater. inasmuch as the pH valuesof the various amine salts used in practicing this invention are withinthe range of from 1 6, it is desirable that these acid-reactingmaterials be added to the water as close to the heater as possible sothat only a small section of the condensate return line is exposed tolowered pH values. To mitigate corrosion by the amine salts in this partof the steam system, any one of several phosphates can be added in anamount ranging from 1-10 P. P. M. as is well known in the art of watertreating. If a heater is not installed in the system, addition of theamine salt can be made conveniently at the sump or hotwell into whichthe condensate return Water is discharged.

The single figure of the drawing with its legends illustrates theapplication of the process as above described.

By using the amine salts, the amine is introduced into the system and atthe same time, since the salts are acid-reacting, they assist in theliberation of carbon dioxide from the amine carbonates and aminebicarbonates in the condensate. Although salts at the upper end of thepH range 1-6 will not assist in liberating carbon dioxide as effectivelyas will the more acidic salts, smaller amounts of strong base would berequired in the boiler to maintain proper alkalinity of the boilerwater. In the table, I enumerate the various stages through which theamine passes in a steam system to which I have added certain aminesalts.

Table 1. Addition of amine salt at hot-well, sump, feed water heater, orother suitable place:

Amine salt is added to the system, lowering the pH and facilitating lossof carbon dioxide from the condensate.

2. Boiler:

Water vaporizes to steam.

Bicarbonates and carbonates are hydrolyzed producing CO2 which passesoil in steam leaving residual alkalinity in boiler water.

Amine salt reacts with strong base present in boiler to produce freeamine and alkalimetal salt, thereby proportionately reducing thealkalinity of the boiler water. Amine is volatilized and mixes withsteam while the alkali-metal salt remains in the boiler.

3. Steam line: I

Steam containing admixed carbon dioxide and amine passes to condensingunit.

il 4. Condensing zone:

Steam condenses to water.

CO: and amine dissolve in water, amine neutralizing to the extentpresent the acidity produced by CO2.

5. Further addition oi amine salt as in step 1:

Amine salt has acid reaction thereby assists in liberation of CO2 fromthe amine carbonates and bicarbonates formed in the system. Also, as thesalt is added, make-up of potential vamine is provided to replenish whatlittle amine is lost from the system.

l In treating steam systems with a volatile alkaline amine to reduceacidic corrosion, it is general practice to add sumcient amine to raisethe pH value of the condensate water to a range of from about 6.0 toabout 8.3. Higher pH values rray be maintained, if necessary, by addingincreased quantities of amine. The pH value may be determined by the useof phenolphthalein or some other suitable indicator added to samples ofcondensate. The greater the amount of acid-reacting amine salt added tothe system and reacted with alkali in the boiler to liberate amine, thegreater will be the pH value of the condensate.

Having introduced an amine salt into the system, subsequent additions ofamine salt are required for a two-fold purpose: to assist in theliberation of CO;` from the system and to replenish what amine may belost from the system. The amount of amine salt required will be governedby the concentration of free amine necessary for protection againstcorrosion in the condensing zone o f the system. Since the aminecarbonates and bicarbonates, which are formed in this zone when the freeamine reacts with the carbonic acid and carbon dioxide present in thesteam, are

relatively unstable at feed-water heater temperatures, some amine andsome carbon dioxide will be lost to the atmosphere. If small amounts oi'the acid-reacting amine salt are added tov replenish the loss of amine,the pH of the condensate will be lowered with a consequent increase inthe partial pressure of the carbon dioxide together with a decrease inthe partial pressure of the amine.

The continuous feed of a very small amount oi amine salt solutionprevents the-loss of excessive amounts of free amine, promotes theliberation of carbon dioxide and continuously introduces a small amountof potential amine to maintain the desired pH value in the condensate.Ii the vcontinuous feed of amine salt is not desired, the salt may befed intermittently when the concentration of amine in the condensingzone becomes too low for eilective corrosion protection.

I may employ a water-soluble salt of any one of a large number of aminesin practicing my invention. For effective inhibition of corrosion in thecondensing zone of the steam system the amine must be suilicientlyalkaline to neutralize the acidity present and it must be suillcientlyvolatile so that it will not remain in the boiler but Will pass overwith the steam into the condensing zone. A suitable measure ofalkalinity is the dissociation constant. Amines having a constant of atleast 1 10 are effective.

The salt of any suitable alkaline amine may be used if that amine ismade available to those parts of the system susceptible to corrosion bythe condensing steam. At a temperature o! C. prevailing in a boiler atatmospheric pressure, any amine boiling at 100 C. or less 5. wouldrapidly volatilize and pass out of the boiler. Salts of the followingamines whose boiling points are less than 100 C. are typical of thecompounds I may use: (figures indicate boiling points at atmosphericpressure) trimethylamine 3.5o C., isopropylamine 33 C., triethylamine89.5 C., and isoamylamine 95 C. I may also use salts of amines boilingat temperatures of less than C.

Many amines having boiling points in excess of that of water atatmospheric pressure may beused to prevent corrosion in steam systems. Anumber of amines having a high boiling point exhibit a suicient vaporpressure to distill with the steam so that salts of amines boiling evenat temperatures greatly in excess of 100 C. may be used in practicing myinvention. For example, tripropylamine boils at 156 C.,trimethylen'ediamine at 135 C., morpholine at 128 C., cyclohexylamine at134 C., and monoethanolamine at 171 C, The water-soluble salts of any ofthese amines may be employed in my process since the amines which areliberated from the salt by the lalkalinity of the boiler will distillwith the steam in effective amount and at a sutilciently rapid rate toprotect the condenser and the condensate return lines from acidiccorrosion.

It should be understood that I may use the water-soluble salt of anyalkaline amine which at the temperatures and pressures encountered willpass from the boiler to the condensing zone of the steam system insufficient concentration to reduce or prevent acidic corrosion. I do notlimit myself to salts of amines boiling in a speci-y fied range butinclude salts of amines which are effective under the conditionsobserved in the operation of a steam system.

The sulfates, suliltes, bisulfates, bisulfites, phosphates. phosphites,nitrates, and hydrochlorides are the preferred salts I employ intreating steam systems. By the term phosphates, I mean to include boththe crystalline phosphates such as orthophosphates, pyrophosphates, andtripolyphosphates, and the amorphous phosphates commonly referred to asphosphate glasses. By reacting eouivalent weights of amine and acid. thesalt is formed. It may be wise under certain conditions tn select aparticular salt the anion of which has properties desirable in watertreating. For example, an amine phosphate would assist in preventingscale formation while an amine nitrate would be useful in preventingembrittlernent of the boiler steel. An amine sulte would inhibit attackof the boiler by dissolved oxygen. In each of these instances, the aminewould be liberated by the alkalinity of the boiler water and would passover to the condensing zone of the system.

Having thus described my invention, what I claim as new and desire tosecure by Letters Patent is:

1. In the process of preventing corrosion in the condensing zone of aclosed condensate return steam system provided with a boiler forsupplying steam thereto wherein a strong base substance is maintained,by means of a volatile alkaline amine, that step which comprisesintroducing into the system at a point between the terminal of thecondensate return line and the boiler a water-soluble acid-reacting saltof an amine which amine has a dissociation constant of at least 1 1UG,thereby promoting the liberation of carbon dioxide from the returncondensate water and reducing the loss of free amine from the system.

2. The method of treating a steam system hav-ling mechanical means bywhich gaseous substances in said system may be evolved to inhibitcorrosion in the condensing zone thereof which comprises introducinginto the system at any suitable point between the condensing zone andthe evaporating zone thereof a water-soluble, acid-reacting salt of avolatile amine, said amine having a dissociation constant of at least 1106 maintaining a strong base substance in' the evaporating zone of thesystem, passing said acid-reacting salt into the evaporating zone Whereit reacts with said strong base to liberate' said amineI loss of theamine is minimized, and passing the; acid-reacting amine salt to theevaporating ZOIle.

3. The method of treating a steam system hav-- ing mechanical means bywhich gaseous substances in said system may be evolved to inhibitcorrosion in the condensing zone thereof which comprises introducinginto the system at any suitable point between the condensing zone andthe evaporating zone thereof a water-soluble acid-reacting salt of avolatile alkaline amine, said amine being suiiiciently alkaline toreduce acidic corrosion in the condensing zone of the system maintaininga strong base substance in the evaporating zone of the system, passingsaid acid-reacting salt into the evaporating zone where it reacts withsaid strong base to liberate the volatile amine, circulating said aminethrough the condensing zone of the system whereby carbonates andbicarbonates of the amine are formed, adding from time to timeadditional acid-reacting salt of the volatile amine whereby carbondioxide is liberated and loss of the amine is minimized and passing theacid-reacting amine salt to the evaporating zone.

4. The method of inhibiting corrosion in the condensing zone of a steamsystem having mechanical means by which gaseous substances in saidsystem may be evolved which comprises introducing into the system at anysuitable point between the condensing zone and the evaporating zonethereof a water-soluble suite of a volatile alkaline amine, said aminehaving a dissociation constant of at least 1 10r6 maintaining a strongbase substance in the evaporating zone of the system, passing said aminesulte into the evaporating zone where it reacts with said strong base toliberate the volatile amine, circulating said amine through thecondensing zone of the system whereby carbonates and bicarbonates of theamine are formed, adding from time to time additional amine suliitewhereby carbon dioxide is liberated and loss of the amine is minimizedand passing the amine sulte to the evaporating zone.

5. The method of inhibiting corrosion in the condensing zone of a steamsystem having mechanical means by which gaseous substances in saidsystem may be evolved which comprises in troducing into the system atany suitable point between the condensing zone and the evaporating zonethereof cyclohexylamine sulfite maintaining a strong base substance inthe evaporating zone of the system, passing said cyclohexylamine sulflteinto the evaporating zone where it reacts with said strong base toliberate the volatile amine, circulating said amine through thecondensing zone of the system whereby carbonates and bicarbonates of theamine are formed, adding from time to time additional cyclohexylaminesulfite whereby carbon dioxide is liberated and loss of the amine isminimized and passing the cyclohexylamine sulflte to the evaporatingzone.

6. As an improved method of introducing a water-soluble volatilealkaline amine into' a steam system having mechanical means by whichgaseous substances in said system may be evolved whereby corrosion inthe condensing zone thereof is reduced, the addition to the system at anappropriate point between the condensing zone and the boiler of awater-soluble, acidreacting salt of the amine maintaining a strong basesubstance in the evaporating zone of the system, passing saidacid-reacting salt into the evaporating zone where it reacts with saidstrong base to liberate the volatile amine, circulating said aminethrough the condensing zone of the system whereby carbonates andbioarbonates of the amine are formed, adding from time to timeadditional acid-reacting salt of the Volatile amine whereby carbondioxide is liberated and lloss oi the amine is minimized and passing theacid-reacting amine salt to the evaporating zone. 7. The method asdescribed in claim 6 where the acid-reacting amine salt is a salt ofcyclohexylamine.

8. The method as" described in claim 6 where the acid-reacting salt isthe sulte of an amine.

GEORGE B. HATCH.

REFERENCES CITED The following references are of record in the file orthis patent:

UNITED STATES PATENTS

